Microgel particles containing methacrylic acid: pH-triggered swelling behaviour and potential for biomaterial application

pH-responsive microgels are crosslinked polymer particles that swell when the pH approaches the pK(a) of the ionic monomer incorporated within the particles. In recent work from our group it was demonstrated that the mechanical properties of degenerated intervertebral discs (IVDs) could be restored to normal values by injection of poly(EA/MAA/BDDA) (ethylacrylate, methacrylic acid and butanediol diacrylate) microgel dispersions [J.M. Saunders, T. Tong, C.L. Le Maitre, T.J. Freemont, B.R. Saunders, Soft Matter 3 (2007) 486]. In this work we report the pH dependent swelling and rheological properties of poly(MMA/MAA/EGDMA) (methylmethacrylate and ethyleneglycol dimethacrylate) microgel dispersions. This system was investigated because it contains monomers that are already used as biomaterials. The poly(MMA/MAA/EGDMA) particles exhibit pH-triggered volume swelling ratios of up to ca. 250. The swelling onset for these particles occurs at pH values greater than ca. 6.0. A pK(a) for these particles of ca. 6.7 is consistent with titration and swelling data. Fluid-to-gel phase diagrams for concentrated poly(MMA/MAA/EGDMA) dispersions were determined as a function of polymer volume fraction and pH using tube-inversion measurements. The rheological properties for the gelled microgel dispersions were investigated using dynamic rheology measurements. The elastic modulus data for the poly(MMA/MAA/EGDMA) gelled dispersions were compared to data for poly(EA/MAA/BDDA) microgels. A similar pH-dependence for the elastic modulus was apparent. The maximum elastic modulus was achieved at a pH of about 7.0. The elastic modulus is an exponentially increasing function of polymer volume fraction at pH 7.0. Preliminary cell challenge experimental data are reported that indicate that gelled poly(MMA/MAA/EGDMA) microgel dispersions are biocompatible with cells from human intervertebral discs. However, the duration over which these experiments could be performed was limited by gradual redispersion of the gelled microgel dispersions. Based on the results presented it is suggested that poly(MMA/MAA/EGDMA) microgel would be a good candidate as a biomaterial for structural support of soft connective tissues.     

Microgels containing methacrylic acid: effects of composition on pH-triggered swelling and gelation behaviours

pH-responsive microgels are cross-linked polymer colloids that swell when the pH approaches the pK _a of the particles. In this work, we present a comprehensive investigation of pH-triggered particle swelling and gel formation for a range of microgels containing methacrylic acid (MAA). The microgels investigated have the general composition poly(A/MAA/X), where A and X are the primary co-monomer and cross-linking monomer, respectively. The primary co-monomers were methyl methacrylate (MMA), ethyl acrylate (EA) or butyl methacrylate. The cross-linking monomers were either butanediol diacrylate (BDDA) or ethyleneglycol dimethacrylate (EGDMA). The microgels were studied using scanning electron microscopy, photon correlation spectroscopy (PCS) and dynamic rheology measurements. Gel phase diagrams were also constructed. The particles swelled significantly at pH values greater than approximately 6.0. It was shown that poly(EA/MAA/X) microgels swelled more strongly than poly(MMA/MAA/X) microgels. Furthermore, greater swelling occurred for particles prepared using EGDMA than BDDA. Concentrated dispersions of all the microgels studied exhibited pH-triggered gel formation. It was found that the fluid-to-gel transitions for the majority of the six microgel dispersions investigated could be explained using PCS data. In those cases, gelation was attributed to a colloidal glass transition. Interestingly, the microgels that were considered to have the highest hydrophobic content gelation occurred under conditions where little particle swelling was evident from PCS. The data presented show that gelled poly(EA/MAA/BDDA) and poly(MMA/MAA/EGDMA) microgel dispersions have the strongest elasticities at pH = 7.     

Study of pH-responsive microgels containing methacrylic acid: effects of particle composition and added calcium

pH-responsive microgel dispersions contain cross-linked polymer particles that swell when the pH approaches the pKa of the ionic monomer incorporated within the particles. In recent work from our group, it was demonstrated that the mechanical properties of degenerated intervertebral discs (IVDs) could be restored to normal values by injection of pH-responsive microgel dispersions (Saunders, J. M.; Tong, T.; LeMaitre, C.; Freemont, A. J.; Saunders, B. R. Soft Matter 2007, 3, 486). These dispersions change from a fluid to a gel with increasing pH. The present work investigates the pH-dependent properties of dispersions of microgel particles containing MAA (methacrylic acid) and also the effects of added Ca2+. Two microgels are discussed: microgel A is poly(EA/MAA/AM) (EA and AM are ethyl acrylate and allyl methacrylate), and microgel B is poly(EA/MAA/BDDA) (butanediol diacrylate). The pH-dependent particle properties investigated include hydrodynamic diameters and electrophoretic mobilities. The critical coagulation concentrations (CCC) of dilute dispersions and the elastic modulus (G') of concentrated, gelled microgel dispersions were also investigated. In the absence of added Ca2+, the particle swelling and G' were smallest and largest, respectively, for microgel A. The changes in hydrodynamic diameter and mobility with pH were explained in terms of a core-shell swelling mechanism. Added Ca2+ was found to significantly decrease the CCCs, extents of particle swelling, and magnitude of the electrophoretic mobility. This was attributed to the ionic cross-linking of neighboring RCOO- groups by Ca2+. It is suggested that the formation of ionic cross-links is inefficient within the microgel particles because of the presence of covalent cross-links that oppose the large-scale conformational rearrangement of neighboring RCOO- groups. The effect of Ca2+ on the properties of the gelled dispersions is important from the viewpoint of potential application in vivo. Rheological studies of the gelled microgel dispersions showed that added Ca2+ did not have a specific influence on G'. The differences observed in the presence of Ca2+ were attributed to ionic strength effects (screening). The key parameter that controls G' of the gelled microgel dispersions is pH. The results from this work suggest that the elasticity of the gels would be slightly reduced in vivo as a consequence of the high ionic strength present.     

A Useful Poly(ethylene oxide)‐poly(propylene oxide)‐poly(ethylene oxide) Triblock Crosslinker in a Diffusion‐Controlled Polymerization Method

Monodisperse micron‐sized polystyrene particles crosslinked with a novel poly(ethylene oxide)‐poly(propylene oxide)‐poly(ethylene oxide) triblock diol diacrylate (t‐BDDA) were produced via simple dispersion polymerization. It was established that the monomer‐diffusible surface characteristics of primary particles played a decisive role in producing the monodisperse crosslinked polymer particles. We named this concept a diffusion‐controlled polymerization method, DPM. Here in this study, particularly, t‐BDDA is proposed as a very useful crosslinker capable of self‐assembling and crosslinking in the process of particle formation and particle growth.     

Effects of pH and salt concentration on oil-in-water emulsions stabilized solely by nanocomposite microgel particles

Aqueous dispersions of lightly cross-linked poly(4-vinylpyridine)/silica nanocomposite microgel particles are used as a sole emulsifier of methyl myristate and water (1:1 by volume) at various pH values and salt concentrations at 20 degrees C. These particles become swollen at low pH with the hydrodynamic diameter increasing from 250 nm at pH 8.8 to 630 nm at pH 2.7. For batch emulsions prepared at pH 3.4, oil-in-water (o/w) emulsions are formed that are stable to coalescence but exhibit creaming. Below pH 3.3, however, these emulsions are very unstable to coalescence and rapid phase separation occurs just after homogenization (pH-dependent). The pH for 50% ionization of the pyridine groups in the particles in the bulk (pK(a)) was determined to be 3.4 by acid titration measurements of the aqueous dispersion. Thus, the charged swollen particles no longer adsorb at the oil-water interface. For continuous emulsions (prepared at high pH with the pH then decreased abruptly or progressively), demulsification takes place rapidly below pH 3.3, implying that particles adsorbed at the oil-water interface can become charged (protonated) and detached from the interface in situ (pH-responsive). Furthermore, at a fixed pH of 4.0, addition of sodium chloride to the aqueous dispersion increases the degree of ionization of the particles and batch emulsions are significantly unstable to coalescence at a salt concentration of 0.24 mol kg(-1). The degree of ionization of such microgel particles is a critical factor in controlling the coalescence stability of o/w emulsions stabilized by them.     

Morphology of core-shell latex particles

The progressive dissolution of carboxylated latex particles with increasing pH was utilized to investigate the internal structure of core-shell latex particles, in comparison with that of copolymeric latex particles formed from the same monomers. The results indicated that in those latex particles which are formed when ethyl acrylate (EA) -methacrylic acid (MAA) or methyl acrylate (MA) -MAA mixture is polymerized in the presence of poly(MA-MAA) or poly(EA-MAA) seeds the shell is composed of the more hydrophilic poly(MA-MAA) molecules relatively high in MAA content and the core is composed of both poly(MA-MAA) and poly(EA-MAA) molecules, regardless of the order of the stage feed, while the copolymeric latex particles are relatively uniform from surface to center in distribution of all components except MAA. Examination of the distribution of the carboxylic groups in all of the latex particles showed their concentration to be highest at the surface and to decrease with proximity to the center in accordance with other findings reported in the literature.     

Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles

Using positively charged plate-like layered double hydroxides (LDHs) particles as emulsifier, liquid paraffin-in-water emulsions stabilized solely by such particles are successfully prepared. The effects of the pH of LDHs aqueous dispersions on the formation and stability of the emulsions are investigated here. The properties of the LDHs dispersions at different pHs are described, including particle zeta potential, particle aggregation, particle contact angle, flow behavior of the dispersions and particle adsorption at a planar oil/water interface. The zeta potential decreases with increasing pH, leading to the aggregation of LDHs particles into large flocs. The structural strength of LDHs dispersions is enhanced by increasing pH and particle concentration. The three-phase contact angle of LDHs also increases with increasing pH, but the variation is very small. Visual observation and SEM images of the interfacial particle layers show that the adsorption behavior of LDHs particles at the planar oil/water interface is controlled by dispersion pH. We consider that the particle-particle (at the interface) and particle-interface electrostatic interactions are well controlled by adjusting the dispersion pH, leading to pH-tailored colloid adsorption. The formation of an adsorbed particle layer around the oil drops is crucial for the formation and stability of the emulsions. Emulsion stability improves with increasing pH and particle concentration because more particles are available to be adsorbed at the oil/water interface. The structural strength of LDHs dispersions and the gel-like structure of emulsions also influence the stability of the emulsions, but they are not necessary for the formation of emulsions. The emulsions cannot be demulsified by adjusting emulsion pH due to the irreversible adsorption of LDHs particles at the oil/water interface. TEM images of the emulsion drops show that a thick particle layer forms around the oil drops, confirming that Pickering emulsions are stabilized by the adsorbed particle layers. The thick adsorbed particle layer may be composed of a stable inner particle layer which is in direct contact with the oil phase and a relatively unstable outer particle layer surrounding the inner layer.     

Formation and Catalytic Functionality of Synthetic Polymer-Noble Metal Colloid

Colloidal dispersions of noble metals in synthetic polymers are prepared by reduction with alcohol. Reflux of a solution of rhodium(III) chloride and poly(vinyl alcohol) (PVA) in a methanol-water mixed solvent under argon or air for 4 hr gives a homogeneous solution of colloidal dispersion of rhodium (Rh-PVA-MeOH/H₂O). The particle size of metallic rhodium is distributed n a narrow range of 30-70 Å, and the average diameter is 40 A. The formation of colloidal rhodium proceeds through three steps: coordination of poly(vinyl alcohol) to rhodium(III) ion, reduction with methanol to form small particles (8 Å in diameter), and growth of the small particle to large particle (40 Å in diameter). Polyvinylpyrrolidone (PVP) and poly(methyl vinyl ether) (PMVE) can be used in place of poly(vinyl alcohol) and result in colloidal dispersions, respectively, similar to Rh-PVA-MeOH/H₂O. Colloidal dispersions in nonaqueous solvent can be prepared by using ethanol instead of methanol-water (Rh-PVP-EtOH) and by using methanol instead of methanol-water, with addition of small amount of methanol solution of sodium hydroxide (Rh-PVP-MeOH/NaOH). The average diameters of rhodium particles in Rh-PVP-EtOH and Rh-PVP-MeOH/NaOH are 22 and 9 Å, respectively. The colloidal dispersions of palladium, silver, osmium, iridium, platinum, and gold in aqueous or nonaqueous solvent are prepared by using polyvinylpyrrolidone. The colloidal dispersions are very stable even under air for 20 days. Those of rhodium, palladium, and platinum are effective catalysts for hydrogenation of olefins at 30°C under an atmospheric hydrogen pressure. The colloidal dispersion of palladium catalyzes highly selective hydrogenation of diene and dienoate to monoene and monoenoate, respectively.     

pH响应性P(MMA-co-MAA-co-HEMA)微凝胶的制备及其性能

利用预乳化乳液法制备了不同单体配比的聚(甲基丙烯酸甲酯-co-甲基丙烯酸-co-甲基丙烯酸羟乙酯)(P(MMA-co-MAA-co-HEMA))微凝胶分散液;采用透射电子显微镜、动态光散射仪研究了微凝胶的微观形态、粒径大小及其溶胀率;利用试管倒转法对微凝胶分散液的凝胶化相转变行为进行了研究,借助椎板流变仪考察了所形成胶态凝胶的储能模量与单体配比、微凝胶分散液浓度和温度的关系.结果表明,所制备的微凝胶的数均粒径为90 nm左右,当MMA与MAA的投料质量不变时,随着HEMA含量的增加,分散液凝胶化所需的临界最小浓度增大,临界最大pH值减小,胶态凝胶的储能模量增加.当保持单体MMA与HEMA的投料质量不变时,随着单体MAA投料质量的增多,微凝胶的数均粒径和溶胀率增大,胶态凝胶的储能模量先升高后降低;当MAA占单体总摩尔数的25%时,浓度为15 wt%的微凝胶分散液在扫描频率为100 rad/s时,胶态凝胶的储能模量最高可达2×104Pa.这类微凝胶分散液在组织工程支架材料方面有潜在的应用价值.     

Study on soap‐free P(MMA‐EA‐AA or MAA) latex particles with narrow size distribution

Soap‐free poly(methyl methacrylate‐ethyl acrylate‐acrylic acid or methacrylic acid) [P(MMA‐EA‐AA or MAA)] particles with narrow size distribution were synthesized by seeded emulsion polymerization of methyl methacrylate (MMA), ethyl acrylate (EA) and acrylic acid (AA) or methacrylic acid (MAA), and the influences of the mass ratio of core/shell monomers used in the two stages of polymerization ([C/S]_w) and initiator amount on polymerization, particle size and its distribution were investigated by using different monomer addition modes. Results showed that when the batch swelling method was used, the monomer conversion was more than 96.0% and particle size distribution was narrow, and the particle size increased first and then remained almost unchanged at around 600 nm with the [C/S]_w decreased. When the drop‐wise addition method was used, the monomer conversion decreased slightly with [C/S]_w decreased, and large particles more than 750 nm in diameter can be obtained; with the initiator amount increased, the particle size decreased and the monomer conversion had a trend to increase; the particle size distribution was broader and the number of new particles was more in the AA system than in the MAA system; but the AA system was more stable than the MAA system at both low and high initiator amount. Copyright © 2006 John Wiley & Sons, Ltd.     

The calcite/water interface II. Effect of added lattice ions on the charge properties and adsorption of sodium polyacrylate

The origin of the surface potential of calcium carbonate in aqueous dispersions and the dissolution of calcite in systems containing excess Ca(2+) and CO(3)(2-) have been the subjects of this study. In addition, stabilization of calcite particles with an anionic polyelectrolyte (sodium polyacrylate (NaPA)) and the effect on surface potential and dissolution of calcite have been studied. Preferential dissolution of either Ca(2+) or CO(3)(2-) from the surface, which is governed by the partial pressure of CO(2) in solution and the pH of the solution, mainly determines the surface potential. Both lattice ions (Ca(2+) and CO(3)(2-)) adsorb onto the surface and thus alter the surface potential. NaPA adsorbs strongly onto the calcite surface regardless of background electrolyte concentration, and reverses the surface potential to negative values. Chelation of the surface due to NaPA can be partly prevented by adding Ca(2+) to the dispersion.     

pH-responsive microgels containing hydrophilic crosslinking co-monomers: shell-exploding microgels through design

pH-responsive microgels are crosslinked polymer colloids that swell when the pH approaches the pK _a of the particles. They have potential application for injectable gels for tissue repair and drug delivery systems. This study focuses on the pH-triggered gelation behaviour of a series of poly (EA/MAA/X) microgels. EA and MAA are ethylacrylate and methacrylic acid. Here, we investigate the effect of crosslinking monomer type (X) on microgel properties. The crosslinking monomers used were poly (ethyleneglycol) dimethacrylate (PEGD), ethyleneglycol dimethacrylate (EGD) and butanediol diacrylate (BDD). The microgel containing PEGD (m-PEGD) is a new system. The microgel containing BDD (m-BDD) was used as a control system. The concentrated microgel dispersions formed physical gels when the pH was increased to 5.3?C6.7, and the polymer volume fractions (? _p ) were above about 0.05. Evidence from photon correlation spectroscopy (PCS) and dynamic rheology was presented for abrupt pH-triggered increases, and then decreases of the hydrodynamic diameters for m-PEGD and the microgel prepared using EGD (m-EGD). This appears to be tuneable through crosslinker structure. An unexpected gelation behaviour, which may involve a new gel state for microgels, was found for m-PEGD dispersions. Uniquely, those dispersions formed gels at pH values less than the microgel's pK _a . This behaviour was linked to an outer-shell electrostatic repulsive interaction. The data point to a phenomenon, whereby the m-PEGD shells appear to explode at pH values above 7.0. The control microgel prepared, using BDD (m-BDD), did not show any evidence of shell fragmentation at any pH. That microgel has potential as a model pH-responsive microgel system in that the properties measured by PCS and rheology agreed well. To probe that system in more detail, the rheological data for m-BDD was analysed using scaling theory. The variation of the storage modulus (G') with ? _p gave a scaling exponent of 2.0.     

The generation of void morphology inside soap-free P(MMA-EA-MAA) particles prepared by seeded emulsion polymerization

Monodisperse soap-free P(MMA-EA-MAA) latex particles were synthesized by seeded emulsion polymerization of methyl methacrylate (MMA), ethyl acrylate (EA) and methacrylic acid (MAA), and the particles with void morphology were obtained after undergoing alkali post-treatment. Effects of treatment conditions on particle morphology were investigated. Results showed that the void particles can be obtained under the conditions of the temperature >60 degrees C, initial pH >10.0, treatment time >20 min and 2-butanone amount >2.0 ml. The particle volume and the void size increased to the maximum and then decreased with the increases of initial pH and the treatment time, and these two values increased monotonously with the treatment temperature or 2-butanone amount increased. When the treatment temperature was elevated to 90 degrees C, the treatment time was longer than 180 min, or the 2-butanone amount was more than 8.0 ml, the relatively small voids inside most of the particles combined together to form a large one. The void structure disappeared completely as the initial pH was higher than 12.0. The generation mechanism of the void morphology was discussed.     

Synthesis and characterization of saccharide‐based latex particles

The synthesis of new polymer colloids based on renewable resources, such as sugar‐derived monomers, is nowadays a matter of interest. These new polymeric particles should be useful in biomedical applications, such as drug delivery, because of their assumed biodegradability. In this work, two new families of polymer latex particles, based on a sugar‐derived monomer, 3‐O‐methacryloyl‐1,2:5,6‐di‐O‐isopropylidene‐α‐D ‐glucofuranose (3‐MDG), were produced and characterized. The syntheses of poly(3‐MDG) crosslinked particles and those obtained by copolymerization with methacrylic acid (MAA), poly(3‐MDG‐co‐MAA) crosslinked particles, were prepared by surfactant‐free emulsion polymerization in a batch reactor. The average particle diameter evolutions, the effect of pH of the dispersion medium on the final average diameters, together with the microscopic and morphological analysis of the particle's surface and inner dominium, were analyzed. Poly(3‐MDG‐co‐EGDMA) stable particles were obtained by adding low amounts of initiator. The surface‐charge density of these particles corresponded to the sulfate groups coming from the initiator. In the second family of latices, poly(3‐MDG‐co‐MAA‐co‐EGDMA) particles, DCP measurements and SEM and TEM observations showed that the sizes and surface characteristics depended on the amounts of MAA and crosslinker used in the reaction mixture. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 443–457, 2006     

Poly(DEAEMa-co-PEGMa): a new pH-responsive comb copolymer stabilizer for emulsions and dispersions

Stimulus responsive copolymers are an important class of surfactants that are attracting growing attention in the literature. When used to stabilize colloids, they confer responsiveness to an otherwise nonresponsive system. In this work, a new pH-responsive comb copolymer surfactant, poly(DEAEMa-co-PEGMa), where DEAEMa and PEGMa are diethylaminoethyl methacrylate and poly(ethylene glycol) methacrylate, is introduced and used to stabilize emulsions and particulate dispersions. The copolymer contained 70 mol % of DEAEMa. Turbidity versus pH measurements and photon correlation spectroscopy of the copolymer solutions revealed pH-triggered collapse of the chains above the pK(a). The surface activity of the copolymer increased with pH. The minimum surface tension measured was 33.6 mN/m at pH = 10. These data enabled identification of the pK(a) for poly(DEAEMa-co-PEGMa) as 6.8. The emulsions consisted of tetradecane-in-water and had a droplet size in the range 5-11 mum. They were slightly flocculated when the pH value was close to the isoelectric point. The emulsions phase separated at low pH values. The particulate dispersions were based on carbendazim, which is a fungacide, and had an average size of 1.8 mum. The data for the emulsions and carbendazim dispersions show that the extent of flocculation decreases with decreasing size of the dispersed phase. Analysis of the data suggest that optimum emulsion stability occurs in the pH region of 5.3-6.8 as judged by turbidity measurements. Electrophoretic mobility measurements as a function of pH for the emulsions and carbendazim dispersions reveal a similar isoelectric point in the range of 8.5-9.0, which is about two pH units higher than the pK(a) of the copolymer. A mechanism that explains the pH-responsive stability of the emulsions and dispersions is presented and discussed.     

The Effect of Poly(methyl vinyl ether-alt-maleic acid) Stabilizer on the Stability of Polyaniline-Poly(methyl vinyl ether-alt-maleic acid) Dispersions

The polyaniline (PANI) dispersions have been prepared in acidic aqueous media by oxidative dispersion polymerization in the presence of a polymeric stabilizer. The polymeric stabilizer used in this study is the poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) which contains acid groups (-COOH). The PANI-PMVEMA particles have a uniform size and a spherical shape. The PANI-PMVEMA dispersions show almost no desorption of the PMVEMA, even though the sonication at 500 W for 20 min and the centrifugation at 500 rpm for 60 min are performed 10 times. The existence of the PMVEMA on the surface is confirmed by X-ray photoelectron spectroscopy. The dispersion stability of the PANI-PMVEMA dispersions is extensively influenced by zeta potential which was governed by the acid group (-COOH) of the PMVEMA on the PANI-PMVEMA particle surface. Copyright 2000 Academic Press.     

Poly(D,L-lactide-co-glycolide) dispersions containing pluronics: from particle preparation to temperature-triggered aggregation

In this work the preparation mechanism, properties and temperature-triggered aggregation of poly(D, L-lactide- co-glycolide) (PLGA) dispersions are investigated. The dispersions were prepared by interfacial deposition in aqueous solution containing Pluronic L62 (EO(6)PO(30)EO(6)) or F127NF (EO(101)PO(56)EO(101)), where EO and PO are ethylene oxide and propylene oxide, respectively. PLGA dispersions were also prepared in the absence of added Pluronic for comparison. The PLGA particles were characterized using SEM, photon correlation spectroscopy and electrophoretic mobility measurements. It was found that the hydrodynamic diameter (d) increased with PLGA concentration used in the organic solvent phase ( C PLGA(o) ). The value for d was proportional to C(PLGA)(o) (1/3). The value for d increased upon addition of 0.04 M NaNO(3) which demonstrated the importance of electrostatic interactions during particle formation. Electrophoretic mobility measurements were conducted as a function of pH and the data used to estimate the Pluronic layer thicknesses on the PLGA particles. The layer thickness was greatest for the PLGA particles prepared in the presence of Pluronic F127NF. PLGA dispersions containing Pluronic L62 exhibited temperature-triggered aggregation in the presence of 0.15 M NaNO(3). It was found that the critical temperature for dispersion aggregation (T(crit)) was comparable to the cloud point temperature ( T(cp)) for the parent Pluronic L62 solution. Conditions were established for achieving temperature-triggered aggregation at body temperature for PLGA particle/Pluronic L62 dispersions under physiological ionic strength and pH conditions. The PLGA/Pluronic L62 mixtures studied may have potential for use as injectable biodegradable implants for controlled release applications.     

Poly(methyl methacrylate-co-ethyl acrylate) latex particles with poly(ethylene glycol) grafts: structure and film formation

Water-based copolymer dispersions were prepared using methyl methacrylate (MMA), ethyl acrylate (EA) (MMA/EA = 1:2), and a series of nonionic polymerizable surfactants, i.e., "surfmers" based on poly(ethylene glycol)-(meth)acrylates. The latexes were compared with the behavior of a conventionally stabilized (nonionic nonylphenol ethoxylate, NP100 with 84 ethylene oxide units) dispersion with the same MMA-EA composition (PMMAEA). A number of techniques were employed in order to characterize structure, dynamics, and film formation properties: solution/solid-state NMR, dynamic/static light scattering (DLS/SLS), differential scanning calorimetry (DSC), tensile/shear mode dynamic mechanical thermal analysis (DMTA), and atomic force microscopy (AFM). The surfmers were found to be miscible with the MMA-EA copolymer at room temperature, with 46-85 mol % of the reacted surfmer detected at the particle surfaces, and the remaining part buried in the particle bulk. In contrast, the NP100 surfactant formed a separate interphase between the copolymer particles with no mixing detected at room temperature or at 90 degrees C. For a 4.0% dry weight concentration, NP100 phase separated and further crystallized at room temperature over a period of several months. Composition fluctuations related to a limited blockiness on a length scale above approximately 2 nm were detected for PMMAEA particles, whereas the surfmer particles were found to be homogeneous also below this limit. On a particle-particle level, the dispersions tended to form colloidal crystals unless hindered by a broadened particle size distribution or, in the case of PMMAEA, by the action of NP100. Finally, a surface roughness (Rq) master plot was constructed for data above the glass transition temperature (Tg) from Tg + 11 degrees C to Tg + 57 degrees C and compared with the complex shear modulus over 11 frequency decades. Shift factors from the 2 methods obeyed the same Williams-Landel-Ferry (WLF) temperature dependence, thus connecting the long-time surface flattening process to the rheological behavior of the copolymer.     

Preparation of Colloidal Transition Metals in Polymers by Reduction with Alcohols or Ethers

Colloidal dispersions of rhodium, palladium, osmium, iridium, and platinum are prepared by refluxing the methanol-water solutions of rhodium(III) chloride, palladium(II) chloride, osmium(VIII) oxide, sodium chloroiridate, and chloroplatinic acid, respectively, in the presence of poly(vinyl alcohol) as a protective colloid. The preparations of colloidal dispersions of rhodium are successful in the presence of vinyl polymer with polar group such as poly(vinyl alcohol), polyvinylpyrrolidone, or poly(methyl vinyl ether). Polyethyleneimine, gelatin, polyethylene glycol), and dextran are ineffective as the protective colloid. Water-soluble primary alcohols such as methanol and ethanol, water-soluble secondary alcohols such as 2-propanol, and water-soluble diethers such as 1,4-dioxane are available as reductants for preparation of the colloidal dispersion of rhodium. The average diameters of metal particles in the colloidal dispersions of palladium, rhodium, platinum, iridium, and osmium in poly(vinyl alcohol) are determined by electron microscopy to be 53, 40, 27, 14, and < 10 Å, respectively. The particle size distribution in each colloidal dispersion is sharp within 50 Å wide. The particles in the colloidal dispersions of both iridium and osmium are highly dispersed with no aggregation, while in the colloidal dispersions of rhodium, palladium, and platinum, there exist aggregates of 5-15, 5-30, and 100-200 particles, respectively. Colloidal dispersions of rhodium, palladium, osmium, and platinum are effective as catalysts for hydrogenation of cyclohexene at 30.0°C under atmospheric hydrogen pressure.     

Colloidal stability of aqueous polymeric dispersions: effect of water insoluble excipients

An important issue in the aqueous coating process is dispersion stability. An unstable dispersion results in aggregation of the colloidal particles, thereby affecting the film coating process. In the coating suspension containing pigment, a latex for aqueous film coating might interact with pigment, resulting in unstable dispersion. We therefore conducted a stability investigation in a mixed dispersion including latexes, EudragitL30D-55 (A-latex), EudragitRL30D (C-latex) and EudragitNE30D (N-latex) and pigments, titanium dioxide and iron oxide yellow. An aggregation of the dispersion containing A-latex was observed at pH 2. Regarding the dispersions with C-latex and N-latex, no aggregation was observed in the range pH 2-11. We calculated total interaction energy between latex-latex particles, pigment-pigment particles and latex-pigment particles based on DLVO theory. The calculated results explained two mechanisms of the stable mixed dispersion. The first was that the individual latex particle and the pigment particle dispersed without aggregation in the mixed dispersion because of the electrostatic interaction. The next was that the latexes adsorbed onto the surface of the pigments, making electrostatically stable heterocoaggregates. We also calculated the binding constant of iron oxide yellow for C-latex at pH 10. The value of the constant was determined to be 1.1 x 10(-2).